Global tablet sales are still well below PC shipments. But one analyst is predicting tablets will overtake PCs by the fourth quarter of this year, or the first quarter of 2013.

Tablet sales are currently about 55-60 million per quarter, while PC sales are in the low 80 million range. But according to analyst Sameer Singh, the tablet market is very similar to the modern smartphone market (Android and iOS phones, as opposed to Windows Mobile, Palm, Symbian, and BlackBerry smartphones), which allows him to compare 16 quarters of phone sales to 11 quarters of tablet sales and make some inferences about when more tablets will be sold than PCs.

The result?

Singh says the crossover point is coming early in 2013, or possibly even earlier:

“The substitution rate patterns between the two markets are remarkably similar, showing a correlation of nearly 93% over the first 11 quarters,” Singh writes. “It is also interesting to note that tablet/PC substitution seems to be happening at a faster pace.”

That faster pace is what makes Singh predict that the switchover point of sales could occur as early as the end of this year, driven by accelerating adoption of cheap tablets in emerging markets. (Which, by the way, sounds a lot like Android, and not much like iPad.)

One thing to note: While phone penetration was fairly limited in the early days of smartphones, the PC market is saturated. Which means that overall growth in the combined PC/tablet market will be in the low single digits, says Singh.

That’s bad news for traditional PC vendors, who are getting eaten at the high end by tough competition in a declining market and at the low end by more nimble new competitors in the tablet market.